Vitamin D deficiency is a problem for the elderly and in individuals with severe liver or kidney disease. Deficiencies may also be noted in individuals who experience little exposure to the sun or have poor nutrition.
Vitamin D refers to a group of closely related compounds of which two are of primary importance in humans, vitamin D2 and vitamin D3. Vitamin D2, ergocalciferol, is depicted in the chemical formula 1 below:

Vitamin D3, cholecalciferol, is depicted in the chemical formula 2 below:

Vitamin D3 is synthesized in the light mediated reactions in the skin and in humans is the most active form. In the liver, vitamin O2 and vitamin D3 are converted to 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3. In the kidney, 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 are converted to 1,25 dihydroxyvitamin D2 and 1,25 dihydroxyvitamin D3.
Vitamin D deficiencies are normally treated with vitamin D2. It is desirable to have an accurate and reproducible method to detect and monitor vitamin D levels and vitamin D2 levels in biological samples to evaluate vitamin deficiencies and treatments. This document will refer to all closely related vitamin D compounds and their precursors and metabolytes as vitamin D analogs. And, when greater chemical specificity is required, the chemical species will be identified by the term vitamin D2 or vitamin D3.